1. Field of the Invention
The present invention relates generally to surface wave transmission systems, and more particularly to a low loss system for launching surface waves over unconditioned lines such as power lines.
2. Detailed Discussion of Related Art
The original mathematical work underlying electromagnetic surface wave theory was done by Maxwell in the second half of the 19th century and is still used today. At the beginning of the 20th century, Sommerfeld and others applied Maxwell's equations to show the possibility of surface waves on a conductor. In the years that followed, further analytical work was done at least as late as in 1941 adding more detail to the theory [Electromagnetic Theory, Stratton, McGraw-Hill p. 27]. None of these theoretical treatments showed how to reduce the theory to practice or how to actually launch a surface wave onto a conductor.
In 1948, in U.S. Pat. No. 2,438,795, Wheeler described an “improved waveguide system” related to more efficiently “translating” signals over a single conductor, such as a power line, or terminating currents flowing on a conductor, particularly an end-fed antenna. This involved improving impedance matching and reducing, but not preventing, radiation from the line or antenna.
In 1954, in U.S. Pat. No. 2,685,068 (hereinafter “Goubau '068”), Goubau showed a practical way to launch and maintain a low loss and non-radiating surface wave on a cylindrical conductor. Referring to both Wheeler and Sommerfeld, Goubau posited:
“Sommerfeld's wave on a bare conductor is constrained to the conductor only by reason of the conductor's finite conductivity” [Goubau '068, column 4, line 26.]
Goubau added and developed a new premise.
“[A] surface wave can be transmitted along a conductor independent of its conductivity by reducing the phase velocity of the same. This reduction in phase velocity can be accomplished by suitably modifying the surface of the conductor.” [Goubau '068, column 4, line 13.]
Goubau further states:
“Any suitable modification of the conductor, or wire, which reduces the phase velocity of the transmitted wave will enable the conductor to be used as a surface wave guide.” [Goubau, column 6, line 61.]
Goubau's surface wave transmission line (SWTL) invention required modification of the conductor in order to reduce the phase velocity of the wave [Goubau '068, column 6, line 61]. Propagation of the wave was initiated onto the conductor by means of a horn launcher [Goubau '068, column 17, line 18].
Goubau taught directly away from the usefulness of uninsulated and unconditioned conductor. He described the potential use of his invention with unmodified conductors and stated:
“Adequate, but less efficient, results for some purposes may be obtained by using a bare, unmodified wire in combination with the launching horn shown in FIGS. 8 and 9. Actually even for a bare conductor there is a microscopically thin dielectric layer present on its surface which tends to concentrate adjacent the conductor the field of the transmitted energy. For frequencies below about 5000 megacycles per second this minute surface layer is insufficient to shrink the radial extent of the field enough to permit the use of a bare conductor with a horn of convenient dimensions. However, at higher frequencies the required thickness of dielectric layer to accomplish a given amount of field concentration is lessened, and use of a bare conductor in combination with a conical horn is feasible. It will be understood that, for any given frequency of the transmitted energy, a considerably larger horn diameter will be required for a bare conductor than for a conductor with modified surface. This is because the shrinkage of the radial extent of the field depends upon the thickness of the dielectric layer on the conductor surface.” [Goubau '068, column 19, lines 10-64.]
Goubau described a system utilizing a quarter wave shorted section, a 3.5 inch cylindrical section and a tapered horn of 22 inches axial length for a total length of greater than 64 cm. He detailed performance measured between 1600 MHz and 4700 MHz and indicated that the flare angle (flare half angle of approximately 16 degrees) was too large for best efficiency at 4700 MHz. [Goubau '068, column 17, lines 53-69.]
In the years that followed, there has been a variety of patents issued related to Goubau's SWTL which was dubbed “Goubau Line” or “G-Line” and is commonly referred to as such in his honor. Goubau made further investigations into his SWTL, related to long distance transmission [Investigation of a Surface-Wave Line for Long Distance Transmission, Goubau, Sharp, Attwood] and described it in comparison to more traditional lines [Open Wire Lines, Goubau] and described the effects of bends [Investigations with a Model Surface Wave Transmission Line, Goubau, Sharp].
By 1964 at least one reference book on electronic and radio theory included descriptions of this SWTL and also referred to it as G-Line [see, Reference Data for Radio Engineers, International Telephone & Telegraph, 11th Printing]. There were several applications of G-Line, but the need for insulation or special conditioning of the conductor generally restricted its use to off-beat problems; transmission to a device being towed from an airplane, communications within a mine and other situations where the expense of installing a specially prepared line was merited.
In 1965 U.S. Pat. No. 3,201,724, to Hafner, described use of Goubau line for transmitting information by way of the electric power grid. This described replacing one of the existing power conductors with a special fabricated conductor, wrapped in copper and insulation, which could be used with special supports to allow launchers to be suitably mounted.
More recently, in 2001 a work described a surface wave method for transporting RF over long distances with low loss using a metalized MYLAR® (dielectric) ribbon [Low-Loss RF Transport Over Long Distances, Friedman, Fernsler]. This referenced previous work but added no new insight into the possibility of SWTL operating on unconditioned lines. This work indicated that without dielectric the wave extends “impractically far” beyond the conductor. [MYLAR is a registered trademark of E. I. Du Pont De Nemours and Company, of Wilmington, Del., and as used herein the term shall mean biaxially-oriented polyethylene terephthalate (boPET) polyester film.]
None of this previous work has recognized a way to separate wave transmission along a single unconditioned conductor from simultaneously causing radiation from this same conductor. Greater and better use of Goubau's invention has been limited by the need for special treatment of the conductor, most often provided by supplying insulation or a special dielectric coating. His invention required this special modification both in order to maintain a non-radiating transmission line and also to reduce the radial extent of the electric field around the conductor in order to allow the use of a horn launcher of convenient size.
The foregoing patent and prior art references reflect the current state of the art of which the present inventor is aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.